This invention relates to the copolymerization of a mono-1-olefin monomer, such as ethylene, with a higher alpha-olefin comonomer.
It is well known that mono-1-olefins such as ethylene can be polymerized with catalyst systems employing vanadium, chromium or other metals on a support such as alumina, silica, aluminum phosphate, titania, zirconia, magnesia and other refractory materials. Initially such catalysts were used primarily to form homopolymers of ethylene. It soon developed, however, that many applications required polymers which were more impact resistant than ethylene homopolymers. Consequently, in order to produce polymer having short chain branching like the more flexible free radical polymerized ethylene polymers, comonomers such as propylene, butene, hexene or other higher olefins were copolymerized with the ethylene to provide resins tailored to specific end uses.
Unfortunately, these comonomers do not efficiently incorporate into the polymer and, therefore, excessive amounts of expensive comonomer must be used. Furthermore, the resultant copolymers are not always linear, low-density copolymers and, thus, are physically weak copolymers, having low impact resistance, especially when made into films. In addition, polymerization operating parameters can create conditions in which the copolymer swells during polymerization and interferes with the polymerization and copolymer recovery processes.